Sunspots and Climate Change

It was exceptionally cold in Europe in February 2012, with temperatures in England dropping to -16°C. That’s colder than some parts of Antarctica (well it is summer down there when it is winter in the UK!). Does this cold weather mean that global warming is a myth? No, not at all, since there is a big difference between weather and climate. The weather (rain, wind, cloud) can change locally hour by hour and day by day. The climate is the average of all these weather characteristics taken over a long period of time.

What evidence do we have for global warming?

The evidence for global warming over the past few decades comes from several different sources:
– the measurements of the global temperature
– the melting of glaciers all over the world
– the breakup of some of the ice shelves in Antarctica
– the dramatic decrease in Arctic Ocean sea ice
– the rise in sea level (10-25cm in the past 100 years)
– the occurrence of extreme weather events
– measurements of ocean heat content
– increased understanding of the underlying physics and an ability to simulate
global climate using computer models

Arctic Ice credit: NASA

Does the Sun heat the Earth?

The Sun provides almost all the heat and energy we have here on Earth. However, conditions near the Earth’s surface are greatly affected by the gases in the atmosphere, in particular water vapour and carbon dioxide. These have the affect of keeping us at a comfortable average temperature of 14°C globally. Without these particular gases in its atmosphere the Earth would be approximately 30°C cooler.

The effect created by the gases is called the ‘Greenhouse effect’, which is explained in more detail on the Sun|trek site.

However, in the past few decades, the amount of carbon dioxide in our atmosphere has increased dramatically. By not allowing enough heat to escape from the Earth, this increase is believed to be the cause of recent global warming.

Has the Earth’s climate changed in the past?

Drilling deep ice cores and examining the layers of gas trapped in the ice, for example in the Antarctic, allows us to monitor the climate far back in time. This gives us information about both the Earth’s temperature and the amount of carbon dioxide in the atmosphere. In the past, the temperature has varied a great deal and there have been long periods of cool temperatures called ice ages. The last ice age ended about 11,000 years ago. These ice ages are believed to be caused by periodic changes in the Earth’s orbit around the Sun.

Antarctic ice core – credit: British Antarctic Survey

Today, however, it is not so much the changes over long periods which worry everyone, but the changes seen in the last 100 years, since this coincides with the increase in industrialisation in much of the world. The 20th century saw a global increase in the surface temperature on Earth of 0.75°C. That may not sound very much, but computer models predict that if it continues at that rate then by the year 2050 the temperature could have increased by 2 or 3°C. If this were to happen, more glaciers and ice shelves could melt and the sea level could rise by more than 30cm. This could have a major impact on many countries, especially low-lying countries like Bangladesh or the Maldives.

Climate change effects

Is the Sun’s radiation constant?

The Sun is not constant in time. It has a cycle of activity. One way this activity can be measured is by counting the number of sunspots. Sunspots were first recorded by astronomers, such as Galileo, in the early 17th century. They have been monitored continuously now for more than three centuries. Find out more about sunspots on the Sun|trek site.

By monitoring the number of sunspots, it was found that the Sun has a cycle of activity lasting approximately 11 years. We are currently heading for a peak in solar activity in the next couple of years (2013/4). When the Sun is active, there are lots of sunspots and solar flares. The X-ray and ultraviolet radiation from the solar atmosphere, the corona, also becomes much stronger. The Sun is being observed daily with satellites from space, such as SoHO, Hinode and the Solar Dynamics Observatory (SDO).

Is there a correlation between solar activity and climate changes?

A plot of solar activity (sunspots) against the surface temperature of the Earth shows that there does seem to be a correlation. In particular, from 1640 until 1700 very few sunspots were observed. Indeed, during those years seeing a sunspot was cause for a great celebration! This period is called the Maunder Minimum, and corresponded to a time of very low temperatures in Europe. Called the Little Ice Age, it was a timewhen rivers and lakes often froze in winter.

Sunspot number variation : Royal Observatory Belgium

How does the Sun influence the Earth’s climate?

The Sun can influence the Earth’s environment and climate in several ways. The total amount of radiation from the Sun reaching the Earth’s atmosphere (called the solar irradiance) varies by a very small amount (only 0.1%) with the solar cycle. However, despite this apparently very small change, historic records (see the graph below) show that the solar irradiance (blue curve) does vary in a manner which seems to correlate with temperature variations on the Earth’s surface (red curve).

Variation in so lar activity (Max Planck Institute) and the Earth's temperature (NASA)

Could the recent temperature increases be caused by the Sun?

Recent global warming, the sharp rise in the Earth’s surface temperature over the past 30 years, cannot be explained by the Sun’s influence. The solar irradiance, although varying with the solar cycle over the past 30 years, has not increased dramatically in the same way that temperatures have.

Are there any processes going on which we don’t yet understand?

Scientists do not yet fully understand the ways in which the Sun might affect the Earth’s climate. Increases in X-ray and ultraviolet radiation, or energetic particles (so-called cosmic rays) from the Sun, might change the chemical composition of the Earth’s atmosphere and so might change the rate or efficiency of cloud formation. Also, the solar wind can prevent the cosmic rays penetrating the Earth’s atmosphere.

Change in X-ray emission over the solar cycle

It is part of the normal process of scientific investigation to debate ideas and to propose theories and possible explanations for the data that are recorded. There are uncertainties in any climate measurements and the computer models used to forecast the climate may not include all the physical effects present in the real atmosphere, but climate scientists make allowance for these shortcomings when they record and interpret the data.

The Sun does influence the Earth’s climate, but the vast majority of scientists are convinced that recent ‘global warming’ is man-made and is due to the increase in the amount of ‘greenhouse gases’ in the Earth’s atmosphere.

Based at the University of Cambridge, UK, I work on solar space missions, SoHO, Hinode and SDO. I am fascinated by the Sun and love to share my enthusiasm with others, particularly school students in the UK and further afield (USA, India, Africa). Sun|trek (www.suntrek.org) has been set up as an educational website about the Sun and its effect on the Earth's environment.